Digital to Analog Converter

The present invention relates to a digital to analog converter (D/A converter) suitable for a complementary metal oxide semiconductor (CMOS) integrated circuit.

A D/A converter has been proposed which comprises a voltage dividing part having a plurality of resistors connected in series between two reference voltage terminals, and a switch array comprising a plurality of switching elements of 2 inputs and 1 output connected between the voltage dividing points of the voltage dividing part and the output terminals, each switching element producing one input voltage according to a supplied digital signal so that a voltage across one voltage dividing point according to the supplied digital signal may be produced. Various semiconductor devices have recently become integrated and D/A converters of the type having this construction are also required to be integrated on a single semiconductor chip. Since many of the semiconductor integrated circuits are CMOS circuits, it is preferable that the switch array part of the D/A converter is constituted by CMOS transmission gates. The transmission gate preferably comprises a CMOS since a MOS transmission gate of a single channel cannot provide a wide range of voltages to be transmitted.

The CMOS transmission gate comprises p-type channel MOSFET (PMOSFET) and n-type channel MOSFET (NMOSFET) connected in parallel between input and output terminals. Digital control signals CS and their corresponding inverted signal CS are supplied to the gates of the respective MOSFETs for simultaneously rendering both MOSFETs conductive or nonconductive for controlling the transmission of voltage from the input terminals to the output terminals. The CMOS transmission gate is defective in that its area on the chip is greater than in the case of an MOS transmission gate of a single channel. A chip of general size (3-5 mm on a side) may not be satisfactorily used for a CMOS D/A converter of, for example, over 5 bits, resulting in higher cost.

The back gate of the NMOSFET is grounded when the CMOS transmission gate is formed on an n-type substrate, and the back gate of the PMOSFET is connected to the power source voltage so that the resistance between the source and drain of the MOSFETs constituting the CMOS transmission gate, that is, the ON resistance, is dependent on the input voltage. The back gate biasing effect of the CMOSFET circuit becomes significant especially when the input voltage is half the power source voltage, and the ON resistance becomes very high. Consequently, the response time becomes long and the operation of the D/A converter may not be of high speed, resulting in inconvenience.

It is, therefore, the object of the present invention to provide a digital to analog converter using CMOS transmission gates wherein the increase in the chip area and the decrease in the response speed due to back biasing effects are eliminated.

In order to achieve this object, the present invention provides a digital to analog converter comprising:

a power source terminal;

first and second reference voltage terminals;

voltage dividing means connected to said first and second reference voltage terminals and having a plurality of voltage dividing points; and

switch array means which is connected to said voltage dividing means and which is formed of a plurality of switching elements each having two CMOS field effect transistor circuits each including a first channel MOS field effect transistor and a second channel MOS field effect transistor whose source and drain are respectively connected to the source and drain of said first channel MOS field effect transistor, each of the output terminals of the current paths of the CMOS field effect transistor circuits of said switching elements being commonly connected, and each of said switching elements being so connected that the voltage across one voltage dividing point of said voltage dividing means is transmitted to the output terminals of one of said switching elements in response to control signals supplied to gates of said MOS field effect transistors.

FIG. 1 is a circuit diagram of a D/A converter in accordance with one embodiment of the present invention;

FIG. 2 is a plan view of an integrated circuit in accordance with this embodiment of the present invention; and

FIGS. 3 and 4 are sectional views along the lines III--III and IV--IV of FIG. 2, respectively.

A D/A converter in accordance with one embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows its circuit diagram. This embodiment is applied to a D/A converter of 3 bits. Digital signal input terminals 10, 11 and 12 are connected to the input terminals of inverters 13, 14 and 15, respectively. The input terminal 10 corresponds to the least significant digit bit, and the input terminal 12 corresponds to the most significant digit bit. The input terminal 10 is connected to the gates of NMOSFETs 16, 17, 18 and 19 and to the gates of PMOSFETs 20, 21, 22 and 23. The output terminal of the inverter 13 is connected to the gates of NMOSFETs 24, 25, 26 and 27 and to the gates of PMOSFETs 28, 29, 30 and 31. A first reference voltage terminal 32 of higher positive voltage is connected to a second reference voltage terminal 42 of lower positive voltage through series connected voltage dividing resistors 33, 34, 35, 36, 37, 38, 39, 40 and 41. In this embodiment, the first reference voltage terminal 32 is at 3 V and the second reference voltage terminal 42 is at 2 V. The resistance of each of the resistors 34 through 40 is R, and the resistance of each of the resistors 33 and 41 is R/2. The value of R is 100 ohms to several kilo ohms. The sources of the NMOSFETs 16, 17, 18 and 19 are connected to the connection point of the resistors 39 and 40, that of the resistors 37 and 38, that of the resistors 35 and 36, and that of the resistors 33 and 34, respectively, and are also connected to the sources of the PMOSFETs 31, 30, 29 and 28, respectively. The drain of the NMOSFET 16 is connected to the drains of the NMOSFET 24 and the PMOSFETs 23 and 31. The drain of the NMOSFET 17 is connected to the drains of the NMOSFET 25 and the PMOSFETs 22 and 30. The drain of the NMOSFET 18 is connected to the drains of the NMOSFET 26 and the PMOSFETs 21 and 29. The drain of the NMOSFET 19 is connected to the drains of the NMOSFET 27 and the PMOSFETs 20 and 28. The sources of the NMOSFETs 24, 25, 26 and 27 are connected to the connection point of the resistors 40 and 41, that of the resistors 38 and 39, that of the resistors 36 and 37, and that of the resistors 34 and 35, respectively, and are also connected to the sources of the PMOSFETs 23, 22, 21, and 20, respectively. Thus, the NMOSFETs 16, 17, 18, 19, 24, 25, 26 and 27 and the PMOSFETs 31, 30, 29, 28, 23, 22, 21 and 20, respectively, constitute the first stage CMOS transmission gates, and the output terminals of these gates are connected in pairs.

The input terminal 11 is connected to the gates of NMOSFETs 43 and 44 and PMOSFETs 45 and 46. The output terminal of the inverter 14 is connected to the gates of NMOSFETs 47 and 48 and PMOSFETs 49 and 50. The sources of the NMOSFETs 47, 43, 48 and 44 are connected to the drains of the NMOSFETs 16 through 19, respectively. The sources of the PMOSFETs 49, 45, 50 and 46 are connected to the drains of the PMOSFETs 20 through 23, respectively. The drain of the NMOSFET 43 is connected to the drains of the NMOSFET 47 and the PMOSFETs 46 and 50. The drain of the NMOSFET 44 is connected to the drains of the NMOSFET 48 and the PMOSFETs 45 and 49. The NMOSFETs 43, 44, 47 and 48 and the PMOSFETs 50, 49, 46 and 45, respectively constitute the second stage CMOS transmission gates, and the output terminals of these gates are connected in pairs.

The input terminal 12 is connected to the gates of an NMOSFET 51 and a PMOSFET 52, and the output terminal of the inverter 15 is connected to the gates of an NMOSFET 53 and a PMOSFET 54. The sources of the NMOSFETs 51 and 53 are connected to the drains of the NMOSFETs 44 and 43, respectively. The sources of the PMOSFETs 52 and 54 are connected to the drains of the PMOSFETs 50 and 49, respectively. The drain of the NMOSFET 51 is connected to the drains of the NMOSFET 53 and the PMOSFETs 52 and 54. The NMOSFETs 51 and 53, and the PMOSFETs 54 and 52, respectively constitute the third stage CMOS transmission gates, and the output terminals of these two gates are commonly connected. The CMOS transmission gates of each stage are thus connected in a tree-like manner.

The back gates of the NMOSFETs 16 through 19, 24 through 27, 43, 44, 47, 48, 51 and 53 are connected to the second reference voltage terminal 42. The back gates of the PMOSFETs 20 through 23, 28 through 31, 45, 46, 49, 50, 52 and 54 are connected to a power source terminal 55. A voltage of 5 V is supplied at the power source terminal 55. The output terminal of the final (third) stage CMOS transmission gates, that is, the drain of the NMOSFET 51 is connected to an analog signal output terminal 56. Although the sources of the MOSFETs are connected to the voltage dividing points of the voltage dividing resistors in the above description, the drains may alternatively be connected. The resistances of the voltage dividing resistors 34 through 40 need not be the same.

The mode of operation of the D/A converter in accordance with this embodiment of the present invention will now be described. Since the input terminal 10 is directly connected to half of the first stage CMOS transmission gates and it is connected through the inverter 13 to the other half, half of the first stage CMOS transmission gates are rendered conductive by a digital signal supplied to the input terminal 10. Thus, the higher or the lower of the two terminal voltages of each of the resistors 34, 36, 38 and 40 are transmitted to the respective output terminals of the first stage CMOS transmission gates, for example, the drains of the NMOSFETs 19, 18, 17 and 16. When a signal of high (H) level is supplied to the gates, the NMOSFETs are rendered conductive and PMOSFETs are cut off. When a signal of low (L) level is supplied to the gates, NMOSFETs are cut off and PMOSFETs are rendered conductive. Assume that a signal of high (H) level is supplied to the input terminal 10. Thus, since the NMOSFETs 16 through 19 and the PMOSFETs 28 through 31 are rendered conductive, the terminals at higher voltage sides of the resistors 34, 36, 38 and 40, i.e., the voltages across the connection point of the resistors 33 and 34, that of the resistors 35 and 36, that of the resistors 37 and 38, and that of the resistors 39 and 40 are connected to the drains of the NMOSFETs 19, 18, 17 and 16, respectively.

The input terminal 11 is directly connected to half of the second stage CMOS transmission gates and it is connected through the inverter 14 to the other half. The second stage CMOS transmission gates are rendered conductive by a digital signal supplied to the input terminal 11. For example, when a signal of H level is supplied to the input terminal 11, the NMOSFETs 43 and 44 and the PMOSFETs 49 and 50 are rendered conductive and the drain voltages of the NMOSFETs 17 and 19 are respectively transmitted to the output terminals of the second stage CMOS transmission gates, e.g., the drains of the NMOSFETs 43 and 44.

Similarly the input terminal 12 is directly connected to half of the third stage CMOS transmission gates and it is connected to the other half through the inverter 15. Thus, half of the third stage CMOS transmission gates are rendered conductive by a digital signal supplied to the input terminal 12. For example, when a signal of H level is supplied to the input terminal 12, the NMOSFET 51 and the PMOSFET 54 are rendered conductive and the drain voltage of the NMOSFET 44 is supplied to the output terminals of the third stage CMOS transmission gates, e.g., the drain of the NMOSFET 51. When signals of H level are supplied to the input terminals 12, 11 and 10 in the order named, the highest voltage among the voltages across the voltage dividing points of the voltage dividing resistors, i.e., the voltage across the connection point of the resistors 33 and 34, is obtained at the output terminal 56. The voltage dividing point which is connected to the output terminal 56 draws nearer to the second reference voltage point 42 every time the digital input signal becomes smaller by 1. When L level signals are supplied to the input terminals 10 through 12, the voltage at the connection point of the resistors 40 and 41 is obtained at the output terminal 56. Since the resistances of the resistors 34 through 40 are the same, the digital to analog conversion of 3 bits is thus accomplished.

The construction of the integrated circuit of this embodiment will now be described. FIG. 2 is a plan view illustrating the integrated pattern of the FETs and the resistors excluding the terminals and inverters of the above embodiment. FIGS. 3 and 4 are sectional views along the lines III--III and IV--IV of FIG. 2, respectively. The same parts are designated by the same reference numerals in FIGS. 1 through 4. A p.sup.+ -type semiconductor region 58 is formed in the surface area of the central portion of an n-type semiconductor substrate 57 to form a diffusion resistor. Both ends of the p.sup.+ -type semiconductor region 58, although not shown, are connected to the first and second reference voltage terminals 32 and 42, respectively. A p-type semiconductor well region 59 as a substrate of NMOSFETs is formed at one side of the p.sup.+ -type diffusion resistor 58 in the surface area of the n-type substrate 57. A p.sup.+ -type semiconductor region 60 is formed in the surface area of the edge portion of the p-type well region 59.

Eight n.sup.+ -type semiconductor regions 61 through 68 are formed as sources of the NMOSFETs 16, 17, 18, 19, 24, 25, 26, 27, respectively, in the surface area of the p-type well region 59 near the p.sup.+ -type diffusion resistor 58. Four n.sup.+ -type semiconductor regions 69 through 72 are formed as sources of the NMOSFETs 43, 44, 47 and 48, respectively, in the surface area of the p-type well region 59 outside the n.sup.+ -type regions 61 through 68. The n.sup.+ -type semiconductor region 69 also functions as the drains of the NMOSFETs 17 and 25. The n.sup.+ -type regions 70, 71, and 72 also function as the drains of the NMOSFETs 19 and 27, 16 and 24, and 18 and 26, respectively. Two n.sup.+ -type semiconductor regions 73 and 74 are formed as sources of the NMOSFETs 51 and 53, respectively, in the surface area of the p-type well region 59 outside the n.sup.+ -type regions 69 through 72. The n.sup.+ -type regions 73 and 74 also function as the drains of the NMOSFETs 44 and 48 and the drains of the NMOSFETs 43 and 47, respectively. An n.sup.+ -type semiconductor region 75 is formed as the drains of the NMOSFETs 51 and 53 in the surface area of the p-type well region 59 outside the n.sup.+ -type regions 73 and 74. The n.sup.+ -type region 75 is connected to the output terminal 56, although it is not shown in the figure.

Fifteen p.sup.+ -type semiconductor regions 76 through 90 are similarly formed as the sources of the PMOSFETs in the surface area of the n-type substrate 57 opposite the p-type well region 59 with the p.sup.+ -type resistor 58 interposed therebetween. The p.sup.+ -type semiconductor regions 76 through 83 as the sources of the PMOSFETs 20, 21, 22, 23, 28, 29, 30, 31, respectively, included in the first stage CMOS transmission gates are in contact with the p.sup.+ -type diffusion resistor 58. As an example, a sectional view along the line III--III through the p.sup.+ -type semiconductor region 80 is shown in FIG. 3. Other p.sup.+ -type semiconductor regions 84 through 90 are not in contact with the p.sup.+ -type diffusion resistor 58. As an example, a sectional view along the line IV--IV through the p.sup.+ -type semiconductor region 86 is shown in FIG. 4. Although it is not shown in the pattern representation of FIG. 2, the surface of the semiconductor region is covered with a silicon oxide film.

The n.sup.+ -type semiconductor regions 61 through 75 as the sources of the NMOSFETs and the p.sup.+ -type semiconductor regions 76 through 90 as the sources of the PMOSFETs for forming the CMOS transmission gates are respectively connected with metal (aluminum) wirings 91 through 105 which cross with the p.sup.+ -type resistor 58. The parts with cross marks in the figures are the parts where the semiconductor and the metal are in contact. The contact parts are formed at the part near the p.sup.+ -type resistor region 58 at the center of the n-type substrate 57 for shortening the aluminum wirings. However, these aluminum wirings are separated from each other by the silicon oxide film 91. The sources of the respective MOSFETs of the first stage CMOS transmission gates are connected to the p.sup.+ -type resistor 58 through the p.sup.+ -type semiconductor region. The sources of the respective MOSFETs of the second and third stage CMOS transmission gates cross the p.sup.+ -type resistor 58 but are not in contact therewith. In a D/A converter having a voltage dividing resistor having a plurality of voltage dividing points, and CMOS transmission gates which are connected to the voltage dividing resistor for connecting a voltage dividing point according to the digital signal supplied to the gates to the output terminal and which are mutually connected in a tree-like manner, the chip area may be utilized efficiently by arranging first channel MOSFETs at one side of the resistor and second channel MOSFETs at the other side of the resistor in a symmetrical manner, and crossing the connection wires of the corresponding FETs with the resistor under insulated conditions. Thus, a D/A converter using CMOS transmission gates is provided with substantially the same chip size as a D/A converter using single channel transmission gates.

Aluminum wirings 106 through 111 as the gates of the respective FETs are formed parallel to the p-type resistor 58. Although not shown in the figure, one end of each of the aluminum wirings 107, 109 and 111 are connected to the input terminals 10, 11 and 12, respectively, and one end of each of the aluminum wirings 106, 108 and 110 are connected to the output terminals of the inverters 13, 14 and 15, respectively. The p.sup.+ -type semiconductor region 60 at the edge of the p-type well region 59 is connected to one end of the p.sup.+ -type resistor region 58 near the second reference voltage terminal 42 with an aluminum wiring 112. Thus, the back gate of the NMOSFET is not at ground voltage but at the second reference voltage (2 V in this embodiment), and the resistance between the source and drain of the NMOSFET is reduced so that the generation of back gate biasing effects may be eliminated. Accordingly, the response time is shortened from when the digital input signal is switched until the predetermined voltage is obtained at the output terminal 56, so that the high speed operation of the D/A conversion is enabled.

Although the diffusion resistor region 58 is p.sup.+ -type in the above description, it may be p- or p.sup.++ -type. Further, the diffusion resistor 58 may be replaced with a polysilicon resistor. Although the p-type well region 59 is connected to the second reference voltage terminal 42, it may alternatively be connected to a part which has a potential up to about 0.7 V higher than the voltage potential at the second reference voltage terminal 42, or to a part with a potential which is lower than the second reference voltage but higher than the ground potential. Although the CMOS transmission gates are formed on the n-type substrate 57, the CMOS transmission gates may be formed on a p-type substrate. In this case, the well region is formed of an n-type semiconductor, and the n-type well region is connected to the first reference voltage terminal 32 of higher positive voltage or a part about the potential of this terminal.